GCSE Geology Igneous Rocks Introduction Click anywhere on
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GCSE Geology Igneous Rocks Introduction Click anywhere on the screen to move on. GCSE Geology
Igneous Rocks GCSE Geology
Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling, crystallization and solidification of molten lava or magma Comprise an interlocking mosaic of crystals GCSE Geology
Extrusive Igneous Rocks Molten rock reaching the Earth’s surface via volcanoes (lava) is termed extrusive GCSE Geology
Intrusive Igneous rocks Molten rock (magma) that solidifies at depth within the lithosphere is intrusive Intrusive rocks may eventually be exposed at the Earth’s surface following a long period of uplift and erosion GCSE Geology
Crystal Size and Cooling Rates Crystal size is determined by the rate of cooling of the magma or lava Instantaneous cooling of lava erupted under water as pillow lavas results in a glassy texture, with no crystals GCSE Geology
Crystal Size and Cooling Rates Rapid cooling in at the earth’s surface over a few months results in crystals of <1. 0 mm in diameter forming lava flows (volcanic) GCSE Geology
Crystal Size and Cooling Rates Minor intrusions-dykes and sills cool slower over thousands of years to form crystals 1. 0 mm-3. 0 mm in diameter GCSE Geology
Crystal Size and Cooling Rates Slow cooling in magma chambers deep underground over millions of years results in larger crystals >3. 0 mm in diameter (plutonic) GCSE Geology
Phenocrysts Large well formed crystals in an igneous rock In Shap Granite the pinkish coloured orthoclase feldspar phenocrysts are up to 3 cm in length GCSE Geology
Groundmass The remainder of the igneous rock made up of smaller crystals In the case of Shap Granite, the groundmass is mainly crystals of biotite mica and quartz GCSE Geology
Phenocrysts and Groundmass Orthoclase phenocrysts up to 6 cm in length Phenocrysts are rectangular Implies 2 stage cooling history 1 cm Finer groundmass 0. 51. 0 mm in diameter GCSE Geology
Equicrystalline Texture All the crystals in the rock are roughly the same size Produced by a steady or constant cooling rate GCSE Geology
Equicrystalline Texture 2 cm Microgranite – even cooling, all crystals 1. 0 – 1. 5 mm GCSE Geology
Porphyritic Texture Large crystals (phenocrysts) set in a finer grained groundmass Formed by two-stage cooling GCSE Geology
Porphyritic Texture-Giant Feldspar Porphyry Phenocrysts up to 5 cm long Long axes of phenocrysts aligned parallel implies flow of magma Groundmass 1. 0 -1. 5 mm GCSE Geology
Vesicular Texture Spherical or ellipsoidal cavities found in lavas Vesicles represent gas bubbles trapped in lava or magma GCSE Geology
Vesicular Texture Vesicles represent trapped gas bubbles in this lava flow Vesicles range from 2 mm to 1. 5 cm in diameter Vesicles are stretched and curved indicating flow of the lava Car key for scale GCSE Geology
Glassy Texture e. g. in Obsidian No crystals visible due to very rapid cooling 1 cm Shows Conchoidal Fracture like glass GCSE Geology
Mineral Content Igneous rocks are classified chemically as Silicic or Mafic according to the main constituent minerals present GCSE Geology
Silicic Igneous Rocks Quartz, Orthoclase Feldspar, Plagioclase Feldspar, Biotite Mica and Muscovite Mica. Rich in silica >66% GCSE Geology
Mafic Igneous Rocks Plagioclase Feldspar, Augite and Olivine Contain less silica 45 – 52% GCSE Geology
The Classification of Igneous Rocks % Silica Content Over 66% 52% to 66% Chemical Group Silicic % Light Minerals 60% or over Intermediate 40%-70% 30%-60% 45% to 52% Below 45% % Dark Minerals 40% or below Mafic Ultramafic GCSE Geology 70%- 90% 10%-30% 90% or over 10% or below Rock Names Rhyolite/Obsidian/Pumice (Fine) Micro-granite (Medium) Granite (Coarse) Andesite (Fine) Micro-diorite (Medium) Diorite (Coarse) Basalt (Fine) Dolerite (Medium) Gabbro (Coarse) - Peridotite (Coarse)
Cornish Granite Glassy, colourless quartz 1 cm All crystals over 3. 0 mm in diameter Black biotite mica with pearly lustre White/creamy plagioclase feldspar GCSE Geology
Shap Granite Porphyritic texture, large phenocrysts and finer groundmass Finer groundmass of quartz and biotite mica 2 -3 mm in diameter Feldspar phenocrysts are well shaped 1 cm Pink coloured orthoclase feldspar phenocrysts up to 3 cm long GCSE Geology
Micro-Granite Formed within the crust in a sill or dyke Mineralogy: quartz, feldspar and mica Equicrystalline texture, all crystals 1. 0 – 1. 5 mm in diameter Formed by an even cooling rate over thousands of years GCSE Geology 2 cm
Gabbro Equicrystalline texture, all crystals roughly similar in size Greenish-black augite Formed deep underground by very slow cooling over millions of years 2 cm Coarse grained, crystals over 3. 0 mm in diameter, suggesting slow cooling GCSE Geology Grey/creamy plagioclase feldspar
Porphyritic Dolerite Medium crystal size mainly 1. 0 – 2. 0 mm Mineralogy: plagioclase feldspar, augite and olivine Groundmass constitutes over 75% of the rock Subhedral phenocrysts of plagioclase feldspar up to 3 mm in diameter Two-stage cooling, finally forming an intrusive dyke or sill GCSE Geology 1 cm
Basalt 1 cm Chilled margin, very fine grained Formed by rapid almost glassy cooling at the earth’s surface over a few weeks or months Mineralogy: plagioclase feldspar, augite and olivine Crystal size is fine, well under 1. 0 mm, (volcanic) GCSE Geology
Peridotite 1 cm Coarse grained >3. 0 mm Ultramafic Low silica content Predominantly olivine Small amount of augite Tiny amount of feldspar Mantle rock GCSE Geology
- Types of igneous sedimentary and metamorphic rocks
- Igneous metamorphic and sedimentary
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- Limestone rock cycle
- Concept map for igneous rocks
- Igneous rock
- Three types of rocks
- Igneous rocks concept map
- Intrusive igneous rocks examples
- Concept map of classification of rocks
- Relative age
- Venn diagram of intrusive and extrusive igneous rocks
- Intrusive vs extrusive igneous rocks
- Intrusive igneous rocks
- Venn diagram of intrusive and extrusive igneous rocks
- Intrusive rocks
- Pumice crystal size
- Extrusive rocks and intrusive rocks
- Ultrabasic rocks
- Is diorite aphanitic or phaneritic